A conventional apparatus for manufacturing semiconductor wafers includes a chamber in which environmental conditions are closely controlled to fixed values suited to the film forming process. During the course of forming thin film wafers, a reactive gas and an inert gas are introduced alternately into the chamber. As the gases are alternately introduced into the chamber, pressure fluctuations occur. The pressure fluctuations cause variations in film thickness and may cause plasma to fail to occur in the chamber. More recent trends toward high integration of semiconductors has led to even thinner films and the pressure variations occurring during the changeover from one gas to the other has made it difficult to control the film thickness to the desired degree.
FIG. 4 illustrates a chamber 20 and prior art apparatus for controlling the feeding of gases into a chamber. The apparatus includes first and second pressure regulators 23, 24, first and second line changeover valves V.sub.1 ' and V.sub.2 ', first and second shunt valves V.sub.3 ', V.sub.4 ' located in vent pipes 28, 29 that are connected to the primary or gas input sides of changeover valves V.sub.1 ', V.sub.2 '. A vacuum gauge 27 is provided to measure the pressure in chamber 20, and a vacuum pump 26 is connected via a vacuum exhaust line 25 and a switch valve V to the chamber 20 to evacuate the Chamber to a set pressure.
Reactive and inert gases are supplied from sources (not shown) to the valves V.sub.1 ', V.sub.2 ' through pressure regulators 21, 23 and feed lines 22, 24. The valves V.sub.1 ', V.sub.2 ' are operated alternately and simultaneously so that the reactive gas and the inert gas are alternately introduced into chamber 20. The valve V.sub.1 ' or V.sub.2 ' that is not open during a specific interval blocks the feed line 22 or 24, respectively. The flow rate of the gases is controlled by the pressure regulators 21, 23, which are mass flow controllers, so that the pressure in chamber 20 may be kept at a set pressure.
Various tests were conducted on the gas feed apparatus of FIG. 4. In these tests, direct touch type metal diaphragm valves were used as the valves V.sub.1 ', V.sub.2 ', V.sub.3 ', V.sub.4 '. The valves were pneumatically operated by controlling the supply of drive air through a solenoid valve (not shown).
Prior to each test, with valves V.sub.1 ' and V.sub.2 ' closed, chamber 20 (measuring 210.times.210.times.210 mm) was evacuated by vacuum pump 26. Next, valve V.sub.1 ' was opened and reactive gas (N.sub.2 at a primary pressure of 1.0 kgf/cm.sup.2 G was supplied to the chamber and pressure regulator 21 adjusted so that the set pressure was established in the chamber. After the pressure in the chamber had stabilized, valve V.sub.1 ' was closed, valve V.sub.2 ' opened, to admit inert gas, and the flow rate of pressure regulator 23 adjusted so that the set pressure was established in chamber 20. The vacuum pressure in the chamber was set at the same value (.+-.0.5%) for the inert gas flow as for the reactive gas flow.
After the preliminary steps set forth above were completed, various tests were carried out using different set pressures in chamber 20, different intervals between changeover of valves V.sub.1 ' and V.sub.2 ' and different conditions of the shunt valves V.sub.3 ' and V.sub.4 ' as set forth in Table I.
TABLE I ______________________________________ Chamber Changeover Test Pressure Vent Interval Results No. (Torr) Line (sec) Shown ______________________________________ 1 5.0 changeover 30,10,5 5A 2 5.0 closed 30,10,5 5B 3 0.2 changeover 30,10,5 6A 4 0.2 closed 30,10,5 6B 5 760, 500, changeover 60,30,10,5 7A 100, 50, 10, 5, 1, 0.2 6 760, 500, closed 60,30,10,5 7B 100, 50, 10, 5, 1, 0.2 ______________________________________